Oh, sorry, for some reason I thought the Duetto had a smaller backing plate than the 15. The product pages say this:
The Rupes LHR 15ES utilizes a 5 inch hook and loop backing plate designed to accommodate 5.5 inch and 6 inch pads
The Rupes LHR 12E Duetto includes a 5 inch backing plate designed to be used with 5.5 and 6 inch pads.
The 5" hybrid pads say this:
The backing material on each pad is 4 inch, which makes it self-centering on the Flex XC3401 VRG Orbital Polisher equipped with the Changeable Backing Plate System. With all other applications, a 3.5 to 3 3/4 inch backing plate is suitable.
I think the pads you meant to buy are the HD Orbital pads:
Lake Country 6 Inch HD Orbital Foam Pads were designed for the Rupes LHR 15ES Big Foot Random Orbital Polisher.
Lake Country HD Orbital 6 Inch Foam Pads